Lumber which has recently been cut contains a relatively large percentage of water and is referred to as green lumber. Prior to being used in construction or other applications which demand good grades of lumber, the green lumber must be dried. Drying removes a large amount of water from the lumber and significantly reduces the potential for the lumber to become warped or cracked. Acceptable water content varies depending on the use of the lumber and type of wood; however, a moisture content of about nineteen percent, or less, is acceptable in many circumstances.
Although lumber may be dried in the ambient air, kiln drying accelerates and provides increased control over the drying process. In kiln drying, a charge of lumber is placed in a kiln chamber. A typical kiln chamber is a generally rectangular building which can be at least partially sealed to control the amount of air that is introduced to and exhausted from the kiln chamber. Further, such kiln chambers typically have reversible cans for circulating heated air through the chamber. The air may be heated in a number of ways, such as by a suspension furnace that exhausts hot air into the kiln chamber, or by heat transfer from steam-carrying pipes that extend through the chamber.
The charge of lumber placed in the kiln chamber typically consists of one or more rectangular stacks of lumber. It is conventional for each stack of lumber to consist of a number of vertically stacked, horizontal rows of lumber that are arranged such that cross-sections of the stack are generally rectangular. The horizontal rows are spaced apart with narrow wooden boards, or the like, referred to as "stickers." The stickers are positioned between each horizontal row to space the rows apart and to allow air to flow between the rows. The stacks of lumber are placed on separate flat-bed cars that are moved upon railroad-type tracks. Kilns may have any desired number of such tracks, and multi-track kilns may accept several stacks of lumber during each drying cycle.
In operation, a charge of green lumber is initially placed in a kiln chamber. After at least partially sealing the chamber, the air within the chamber is heated to facilitate drying. The fans within the chamber circulate the heated air through the kiln chamber. Because the stickers provide spaces between the horizontal rows of lumber, the heated air passes between the rows of lumber and is in direct contact with both the upper and lower surfaces of individual pieces of lumber so that the lumber is dried.
FIG. 1 is a perspective view of a conventional stack of lumber 10 that is to be dried in a kiln in the manner generally described above. More specifically, the stack 10 includes a first side 12 and an opposite second side 14, and multiple horizontally extending layers 16 of lumber that arc arranged one above the other and extend between the first and second sides. Each layer 16 includes multiple pieces of lumber 18. Multiple stickers or spacers 20, which are typically in the form of narrow pieces of lumber, are positioned between the layers 16 and extend between the opposite sides 12 and 14, so that multiple passages 22 are defined between adjacent layers 16 and are open at the opposite sides. Only a few of the layers 16, pieces of lumber 18, spacers 20 and passages 22 are identified with a reference numeral in FIG. 1. The stack 10 is positioned within the chamber of a kiln, and heated air is circulated in the chamber so that a flow of heated air is forced through each of the passages 22.
A representative passage 22 is best seen in FIG. 2, which is a cross-sectional view of a portion of the stack 10 taken along line 2--2 of FIG. 1. FIG. 2 diagrammatically illustrates boundary layers 24 that form while airflow is forced into the passages 22 via openings of the passages that are at the first side 12 of the stack 10. The direction of the airflow is generally designated by the arrows 23 in FIG. 2.
Each of the passages 22 of the stack 10 are generally identical; therefore, the flow into the passage 22 that is illustrated in FIG. 2 is generally representative of the flow into each of the passages 22 via the openings to the passages that are at the first side 12 of the stack 10. Whereas FIG. 2 has been described heretofore as being illustrative of airflow into the passages 22 via openings at the first side 12 of the stack 10, FIG. 2 is also illustrative of airflow into the passages via openings at the second side 14 of the stack, in which case FIG. 2 is a cross-sectional view of a portion of the stack taken along line A--A of FIG. 1.
As best seen in FIG. 2, for each of the passages 22, airflow therethrough is such that viscous layers of air are developed proximate to the surfaces of the pieces of lumber 518 that face and define the passage. Those viscous layers are referred to as boundary layers 24, which are not visible but are generally shown in dashed lines in FIG. 2. More specifically, the boundary layers 24, which are areas of retarded flow, are caused by the viscous interaction between the airflow through the passage 22 and the surfaces of the pieces of lumber 18 that define the passage, as well as interaction between the airflow and the lumber surfaces that are proximate to the inlet opening of the passage.
Each boundary layer 24 includes a protruding portion 26 that tapers to a generally planar portion 28. For each of the boundary layers 24, the protruding portion 26 is a portion of the boundary layer that has become separated from the surface or surfaces of the one or more pieces of lumber 18 that define the passage. The separation occurs because of interaction between the airflow and an edge or edges of the one or more pieces of lumber 18 that define the inlet to the passage.
As illustrated in FIGS. 1 and 2, it is conventional for the edges of the layers 16 to be aligned so that they extend in a common plane. As a result, for each of the passages 22, the protruding portions 26 of the boundary layers 24 are aligned in a manner that is very restrictive to flow, since the boundary layers are regions of retarded flow and thereby tend to block flow into the passage 22. More specifically, an unrestricted flow path exists only in that region between the boundary layers 24 of each of the passages 22. Those unrestricted flow paths are characterized by generally inviscid flow. However, within each passage 22, the protruding portions 26 are aligned to significantly restrict the flow such that the only unrestricted flow path is between the peaks of the protruding portions, as designated by the arrow 30 in FIG. 2.
The resistance to flow through the stack 10 that results from the alignment of the protruding portions 26 reduces the speed at which the pieces of lumber 18 can be dried, which can be disadvantageous. The resistance to flow through the stack 10 that results from the alignment of the protruding portions 26 also requires significant pressure increases to maintain the flowrate; therefore, the kiln fans, which force the airflow through the stack, must work excessively, which is disadvantageous.